The efficient and catalytic reduction of carbon dioxide, CO2, by oxygenic photosynthetic organisms has inspired a variety of sustainable energy and chemical-feedstock applications because CO2 is a cheap, abundant, and stable molecule. CO2 is also a greenhouse gas, so its fixation as part of a carbon-neutral energy cycle would significantly decrease the environmental risks associated with greenhouse gases. Obtaining methane or methanol by the chemical reduction of CO2 is desirable, but the strenuous kinetic requirements have thus far been prohibitive.
The inventors have therefore focused their attention on the partial reduction of CO2 to formic acid as an energy-storage medium, because formic acid is able to store H2 in liquid form and is a net carbon-neutral fuel, as shown below in Scheme 1. Hydrogen-storage media are also increasing in importance as research on H2-releasing processes such as water oxidation and artificial photosynthesis intensifies.

Water is an attractive solvent because it is benign and because the aqueous reaction is exergonic (ΔG°298=−4 kJmol−1 at pH=7), while it is endergonic in the gas phase (ΔG°298=+33 kJmol−1). However, despite several decades of research nearly all reported catalytic systems require extreme temperatures and pressures as well as organic additives to achieve satisfactory turnover numbers and reaction rates for both reaction directions.
Thus, there is a need for a catalyst that provides satisfactory turnover numbers and reaction rates for both reactions and does not require extreme temperatures and pressures or the presences of organic additives.